It seems Fire Emblem Heroes might be consuming a lot of resources (RAM, CPU) in addition I suspect it has one or a few processes running in the background.
I'd suggest you completely disable Fire Emblem Heroes from auto starting, manage ram by hibernating the app after using it. You can put the app on the exclusion list of allowed app when screen is off, if your ROM supports that.
For instance I use Start up manager to prevent a named app from automatically running during start up, and Power manager to restrict background power intensive app from running (both built-in)
If your ROM doesn't include these features you may consider installing Greenify, and clearing cached data regularly